The US has accused Iran of "aiding and abetting" the massacre of women and children in Houla by providing support for the Syrian militia accused of carrying out the slaughter.

The state department said that Tehran was "bragging" about its role at the weekend after the deputy commander of Iran's elite Quds force said the force has units inside Syria in support of President Bashar al-Assad.

Victoria Nuland, the state department spokeswoman, said that Iran's hand is clearly visible in the killing of more than 100 people, including scores of young children, by a Syrian militia group, the Shabiha, which closely resembles an Iranian militia, the Basij.

"The Iranians have clearly supplied support and training and advice to the Syrian army, but this Shabiha thug force mirrors the same force that the Iranians use. The Basij and the Shabiha are the same type of thing and clearly reflects the tactics and the techniques that the Iranians use for their own suppression of civil rights," she said.

"It was Assad and his regime that created this Shabiha force to begin with. It very much models the Iranian Basij model where they hire young guys to indiscriminately wreak vengeance and do this kind of hand-to-hand violence."

Nuland further implicated Iran by drawing attention to the timing of a claim by the deputy head of its Quds force, Esmail Ghani, that it has played a "physical and non-physical" role in Syria, and that if it were not present "the killing of citizens would be greater".

"We find it interesting that it was on this very weekend that the deputy head of the Quds force decided to take credit for the advice they're giving to Syria," she said.

The US claim of Iranian involvement came as Washington joined Britain, France, Canada and other nations in expelling top Syrian diplomats over the Houla massacre. The Syrian charge d'affaires, Zuheir Jabbour, was given three days to leave the US. The Syrian ambassador left Washington in December and has not been replaced.

"We took this action in response to the massacre in the village of Houla – absolutely indefensible, vile, despicable massacre against innocent children, women shot at point blank range by regime thugs, the Shabiha, aided and abetted by the Iranians who were actually bragging about it over the weekend," said Nuland.

The US's attempts to hold Iran responsible in part for the Houla massacre is likely to be seen as interwoven with Washington's attempts to pressure Tehran over over its nuclear programme.

But the diplomatic expulsions fall well short of the action demanded by some members of Congress and human rights groups.

Mitt Romney, the Republican presidential candidate, said he welcomed the move but called on Barack Obama to be more assertive and arm the Syrian opposition.

"President Obama's lack of leadership has resulted in a policy of paralysis that has watched Assad slaughter 10,000 individuals. We should increase pressure on Russia to cease selling arms to the Syrian government and to end its obstruction at the United Nations. And we should work with partners to arm the opposition so they can defend themselves," he said.

On Sunday, General Martin Dempsey, the chairman of the US joint chiefs of staff, warned that Syria could face military intervention if the killings of civilians continued.

But the White House on Tuesday rejected armed involvement either directly or by supplying weapons to the regime's opponents, saying that it "could make it harder to achieve the political transition that the Syrian people deserve".

Jay Carney, the White House spokesman, said: "We do not believe that militarisation, further militarisation of the situation in Syria at this point is the right course of action. We believe that it would lead to greater chaos, greater carnage."

He added that calls for the US to replicate the military action it took with Nato during the uprising against Muammar Gaddafi in Libya last year failed to take account of a very different situation in Syria.

"The comparison with I think Libya is one that requires a great deal of recognition of the differences between the two. And as you know, in a unique situation with regards to Libya, with a broad international consensus, a request for military action from the opposition, and the fact that forces loyal to Qaddafi were on the verge of assaulting an entire city and with the promise of annihilating residents of that city, we were able to act with our allies to prevent that," he said.

"Every situation within the Arab Spring and the circumstances is different as we've said on numerous occasions, and certainly the situation in Syria is different from what it was in Libya."

Nuland said the US government recognises that expelling diplomats only has a limited impact.

"Obviously this is a political measure. This is a statement of our extreme disapproval and horror at the massacre. We will obviously continue to look at other ways we can pressure the regime economically, politically, diplomatically and continue to try to tighten the noose," she said.

US officials say that Washington is constrained on the UN security council by Russia's opposition to stronger measures against Damascus. However, Moscow has said it does support an impartial investigation of the Houla killings. Nuland said the US hopes that is indicative of a greater willingness by Russia to permit greater international pressure to be brought to bear on Damascus.

"We are appreciative of the fact that the Russians are willing to have a full investigation because we think it's indisputable what that investigation's going to show. It's going to show that these were regime-sponsored thugs who went into villages, went into homes, and killed children at point blank range and their parents," she said. "Is this going to be a turning point in Russian thinking? We hope so."

Amnesty International has called on the UN security council to move beyond condemnation and refer the actions of the Syrian government to the International Criminal Court for investigation.

The human rights group said it has collected the names of more than 1,300 people killed in Syria since the start of the UN observer mission on 14 April.

Amnesty International said it had little confidence in the Syrian government announcement that it will set up a "judicial military committee" to investigate the killings.

"There is always a military option," he told Fox News. "You'll always find military leaders to be somewhat cautious about the use of force, because we're never entirely sure what comes out on the other side. But that said, it may come to a point with Syria because of the atrocities."

But in his Memorial Day address, Obama said he would not be rushing into other conflicts after Afghanistan and Iraq "unless it is absolutely necessary".